Method for preparing an organo-silicon sol and materials obtained from such sol

ABSTRACT

The method consists in hydrolysing an initial volume Vsi of a precursor material comprising at least one polyalkoxysilane with a quantity of water such that  
           x                   H   2        O       x      Si       ≥   10                 
 
     where x H 2 O and x Si represent the number of moles of H 2 O and Si present, respectively, the concentration of the hydrolysate up to a volume substantially equal to the initial volume Vsi, leaving the concentrated hydrolysate until segregation into an aqueous phase and an organo-silicon phase, and recovery of the organo-silicon phase.  
     Application to optic.

[0001] This invention relates generally to a method for preparing ahydrolysate of organoalkoxysilanes, in particular a hydrolysate in whichthe hydrophobic organic and low polar molecules exhibit excellentsolubility, as well as the use of this organoalkoxysilane hydrolysatefor obtaining transparent films or substrates, including or not organicmolecules and/or inorganic particles, and the applications of thesesubstrates and films in the field of optics, in particular ophthalmicoptics.

[0002] Below in this request, the organoalkoxysilane hydrolysate will becalled organo-silicon sol.

[0003] Generally, preparing organo-silicon sols is difficult.

[0004] The final properties of the sol and consequently of the derivedsubstrates and/or transparent films depend to a large extent on the solpreparation method, even if the final sol drying/condensation step forobtaining the substrate and/or the film also plays a significant role.

[0005] Such sols should exhibit stability properties, i.e. after itspreparation, the essential characteristics of the sol (condensationrate, proportion of the various hydrolysed and/or precondensed species,viscosity) do not change or very little with time.

[0006] Besides, in the optical field, it has also be sought to obtainorgano-silicon sols capable of solubilising low polar, hydrophobicorganic additives, in particular for obtaining films of a few microns inthickness.

[0007] This latter property of the sol must be preserved during thedrying step, i.e. when eliminating solubilisation solvents and duringfinal condensation of the species derived from hydrolysis, so that theadditive does not precipitate during this step.

[0008] Among organic additives that are particularly interesting in theoptical field, photochromic compounds can be mentioned.

[0009] The document FR-A-2 704 851 describes a method for preparing anorgano-silicon sol in which the following operations are conducted:complete hydrolysis of a solution containing one or severalorgano-alkoxysilanes in an organic solvent or mixtures of organicsolvents using an acid aqueous solution with a pH equal to or smallerthan 3, elimination of the organic solvent(s) and of the residualalcohols and concentration of the solution by distillation for obtaininga sol.

[0010] However, the method of the patent FR-A-2 704 851 leads to solscertain properties of which strongly vary with time, in particular thecondensation rate and the composition of the species present in the sol.

[0011] Moreover, it is difficult to solubilise in the sols of patentFR-A-2 704 851 low polar, hydrophobic organic additives and inparticular photochromic compounds.

[0012] The article entitled “Organosiloxane Resin with High SilanolContent” Furuya et al.—Silicones in Coatings II—A Technology ForumExploring the Versatility of Silicone—Mar. 24-26,1998—Florida—USA—Conference Papers”, describes the synthesis of anorganosiloxane resin by hydrolysing trialkoxysilanes with acidifiedwater in the absence of an organic solvent. The alcohol produced duringhydrolysis is eliminated by heating or under reduced pressure in orderto precipitate a viscous product that is a siloxane resin with highsilanol content.

[0013] Although the method of the article leads to more stable sols, itwould be nevertheless desirable to obtain sols with increased stabilityas well as better solubility of additives such as photochromiccompounds.

[0014] It has been found according to the invention that by hydrolysingan organo-silicon precursor with large water excess, then byconcentrating the hydrolysate and by leaving it until segregation intoan aqueous phase and an organo-silicon phase, and by dispersing againthe collected organo-silicon phase having a very low water content, andpossibly dried, in a hydrophobic solvent, a very stable sol could beobtained, in which it was possible to solubilise additives such asphotochromic compounds.

[0015] According to the invention, the method for preparing anorgano-silicon sol comprises

[0016] a) hydrolysis of an initial volume V_(si) of a precursor materialcontaining at least an organo-silicon monomer precursor with formula:

R¹ _(n)Si (OR²)_(4-n)  (I)

[0017] in which

[0018] the radicals R¹, identical or different, represent an alkylgroup, an aryl group, a vinyl group or H,

[0019] the radicals R², identical or different, represent H or an alkylgroup, and

[0020] n is an integer varying from 1 to 2,

[0021] n=2 if R¹ represents H,

[0022] with a water quantity such as$\frac{x\quad H_{2}O}{x{Si}} \geq 10$

[0023] and, with a possible quantity of an organic solvent such that$0 \leq \frac{x{Solvent}}{x{Si}} \leq 8$

[0024] where x H₂O, x Si and x Solvent represent, respectively, thenumber of moles of H₂O, Si and Solvent present.

[0025] and under the condition that when${\frac{x\quad H_{2}O}{x{Si}} = 10},{{x{Solvent}} = 0},$

[0026] to obtain a hydrolysate of the precursor material;

[0027] b) concentration of the hydrolysate down to a volumesubstantially equal to the initial volume V_(si);

[0028] c) leaving the concentrated hydrolysate until a distinct aqueousphase and a distinct organo-silicon phase are obtained, and

[0029] d) separation and collection of the organo-silicon phase. Therecovered organo-silicon phase is preferably subjected to a drying step(e), either (1) by addition of a solvent with a boiling point above 100°C. at atmospheric pressure or a solvent forming an azeotrope elementwith water (for example 2-butanone Teb≈79.6° C.) and evaporation of thesolvent, or (2) by extraction with a hydrophobic solvent.

[0030] Using a solvent with a boiling point greater than 100° C. callingfor heating at relatively high temperature in order to eliminate thesolvent, has the shortcoming of causing the soil (condensation rate) toevolve.

[0031] Azeotropic distillation, although resorting to lowertemperatures, calls for repeated distillations and the quantity of waterremaining in the sol remains relatively important.

[0032] It is therefore preferable to dry by extraction with ahydrophobic solvent exhibiting a boiling point equal to or smaller than80° C. Preferably, ethyl acetate or diethyl ether is used.

[0033] The recommended drying method is diethyl ether extraction that,however, implies replacing ether with another solvent.

[0034] Indeed, diethyl ether is not a solvent appropriate for usage ofthe sol. The sol is polar and its solubility in diethyl ether does notenable to achieve the volume V_(si) by evaporation. Moreover, thisvolatile solvent does not enable the shaping of materials.

[0035] Diethyl ether can be replaced easily with any solvent with higherboiling point and in which organo-silicon species are soluble.

[0036] Diethyl ether is therefore evaporated partially under reducedpressure (down to the solubility limit of the sol), the replacementsolvent is added in excess (for example 2 V_(si)), then the evaporationis carried on until the volume V_(si) is obtained.

[0037] This latter operation is conducted twice in order to evaporateall the diethyl ether present in the sol.

[0038] The solvents used are, for instance, acetone, 2-butanone,tetrahydrofuran.

[0039] When the diethyl ether extraction step is used both the obtainedsilicon organic phase and aqueous phase can be extracted with ether andboth ethereal phases are gathered before replacing ether with anothersolvent.

[0040] Hydrolysis water is an aqueous solution with a pH ranginggenerally between 3 and 10, and preferably acid. The hydrolysis solutioncan be acidified by an inorganic acid such as HCl, HNO₃ or H₂SO₄ or anorganic acid such as acetic acid.

[0041] As stated above, the quantity of aqueous solution used forhydrolysis is such that the following ratio$\frac{x\quad H_{2}O}{x{Si}} \geq 10$

[0042] preferably $10 \leq \frac{x\quad H_{2}O}{x{Si}} \leq 20$

[0043] The hydrolysis medium may comprise an organic solvent selectedpreferably among THF (tetrahydrofuran), inferior alcohols such asethanol or inferior ketones such as acetone.

[0044] The hydrolysable precursor material comprises at least oneorgano-silicon monomer precursor with the following formula:

R¹ _(n)Si (OR²)_(4-n)  (I)

[0045] where R¹, R² and n are such as defined previously.

[0046] R¹ represents preferably a methyl, ethyl, phenyl radical or aphenyl radical substituted with preferably non-polar groups (for examplealkyl groups such as methyl, ethyl, propyl, or phenyl groups or still avinyl radical.

[0047] R² represents preferably H or a C₁ to C₇ alkyl group, fbr examplea methyl, ethyl or propyl radical.

[0048] Preferably, n is equal to 2 to 1, and ideally n is equal to 1.

[0049] Among the particularly preferred organo-silicon precursors withformula (I), the following can be mentioned: methyltrimethoxysilane(MTMOS), methyltriethoxysilane (MTEOS), ethyltriethoxysilane (ETEOS),dimethyldimethoxysilane (DMDMOS), dimethyldiethoxysilane (DMDEOS),diethoxymethylsilane (HMDEOS), phenyltriethoxysilane (PTEOS) andvinyltriethoxysilane (VTEOS).

[0050] Apart from a monomer precursor or a mixture of monomer precursorswith formula (I), the precursor material may comprises at least onemonomer precursor selected among the epoxytrialcoxysilane monomers.Among these epoxytrialcoxysilane monomers, silanes with epoxy group ofthe following formula can be mentioned:

[0051] in which:

[0052] R is a C₁-C₆, preferably CH₃ or C₂H₅, alkyl group,

[0053] R′ is a methyl group or a hydrogen atom,

[0054] a is an integer from 1 to 6, and

[0055] b is equal to 0, 1 or 2.

[0056] The preferred epoxysilanes are γ-glycidoxypropyltrimethoxysilaneor γ-glycidoxypropyltriethoxysilane.

[0057] The particularly preferred monomer isγ-glycidoxypropyltrimethoxysilane (GLYMO).

[0058] Generally, the resting time to obtain the phase separation(segregation) may vary from 1 to several days up to several weeks, forexample 4 to 6 weeks.

[0059] To obtain a phase separation (segregation) during the restingstep (c), when the precursor material comprises a monomer precursor offormula (I) and an epoxytrialkoxysilane monomer precursor, theproportion in molar percentage, of epoxytrialkoxysilane monomerprecursors in relation to the monomer precursors of formula (I), isgenerally in the order of 50% or less according to the monomers used.Preferably, the molar proportion of epoxytrialkoxysilane monomerprecursors with respect to the monomers of formula (I) will be approx.25% or less. The elimination step (b) of water and organic solvents canbe conducted by any appropriate means, but preferably by application ofa primary vacuum.

[0060] According to the application contemplated for the organo-siliconsols according to the invention, additives can be introduced to modifythe mechanical (elasticity, rigidity, hardness) or optical (index,colour) properties of the end-product, for example by addition of anadditive as an organic solution compatible with the organic medium ofthe sol, then concentration of the sol.

[0061] The additives used can be colouring agents such as lasercolouring agents, enzymes, semiconductor or magnetic nanoparticles orstill photochromic compounds.

[0062] The structure of the sols according to the invention, exhibitinghigh rate of units T², proves particularly suited to promotespectrokinetic performances of the organic photochromic compounds.

[0063] The photochromic compounds used preferably are spirooxazines,chromens or fulgides.

[0064] There can also be incorporated in the organo-silicon phase or solaccording to the invention a predetermined quantity of colloidal silica,preferably colloidal silica in an organic solvent whose pH rangesbetween 3.5 and 6, such as the mixed colloidal silica SiO₂/Al₂O₃ (pH 5)in a quantity representing up to 60% by weight of the sol.

[0065] The colloidal silica is generally introduced in the final solpreparation step, after recovery of the organo-silicon phase.

[0066] The organo-silicon sols according to the invention arecharacterised, among other things, by a stable condensation rate (Tc),equal to or greater than 0.65 and the presence of a molar content ofsilicon units T² greater than or equal to 50%, and preferably greaterthan or equal to 60%.

[0067] Preferably, the organo-silicon sols according to the inventionare deprived of water, as determined by the absence of peakscorresponding to water by RMN ¹H.

[0068] The invention also relates to organo-silicon sols with theprevious features and including at least one solubilised photochromiccompound.

[0069] It has been determined that the solubility of the additives, inparticular of the photochromic compounds, was vastly increased with theorgano-silicon sols according to the invention.

[0070] Besides, the organo-silicon sols according to the invention arevery stable.

[0071] The organo-silicon sols according to the invention can then beshaped and condensed into massive materials such as xerogels or intothin films.

[0072] The sols according to the invention are particularly suited tothe realisation of anti-abrasion hard films or anti-reflection films inthe field of ophthalmic optics, and especially for spectacle glasses.

[0073] The following examples illustrate the present invention In theexamples, unless other stated, all the percentages and parts areexpressed in weight.

[0074] Example of Preparation of an Organo-Silicon Sol According to theInvention

[0075] An example for preparing an organo-silicon sol will now bedescribed while referring to FIGS. 1a to 1 b that representdiagrammatically the various steps of the method according to theinvention.

[0076] Hydrolysis of the Precursors

[0077] To a molar equivalent of methyltriethoxysilane (MTEOS) that, asshown on FIG. 1a, represents an initial volume V_(si), is added 20 watermolar equivalents whose pH has been lowered to 3.8 by addition of HCl.

[0078] The mixture is stirred for 15 hours.

[0079] Initially, the mixture is not miscible. When the progress of thehydrolysis is sufficient (2 hours), the ethanol formed enablesmiscibility. The solution is then limpid.

Concentration of the Sol

[0080] After hydrolysis, the solvents are evaporated under reducedpressure (preferably a primary vacuum) until the initial volume V_(si)is obtained.

[0081] Segregation

[0082] The sol thus obtained is stored at 4° C.

[0083] After 24 hours, the MTEOS sol is cloudy and segregation starts.

[0084] After 5 days at 4° C., maximum segregation is achieved and asshown on FIG. 1b, a very viscous organo-silicon phase and an aqueousphase floating on the surface are obtained.

[0085] The composition of both phases has been obtained by RMN ¹H.

[0086] The relative proportions of each specie are summed up in thetable below (molar proportions). Species Silicon phase Aqueous phaseCH₃—Si 46%  0.6% CH₃CH₂OH  4%  1.4% H₂O 50% 98%

[0087] Condensation rate (Tc) of the silicon species in the siliconphase 0.78

[0088] Condensation rate (Tc) of the silicon species in the aqueousphase 0.66

[0089] At that stage, the silicon phase comprises 50% water.

[0090] This phase is collected, then treated with diethyl ether.

[0091] The water fraction trapped previously in the silicon phase isclearly visible and can be separated without any difficulty.

[0092] The spectrum RMN ¹H of the silicon phase dispersed again in thediethyl ether does not show any peaks corresponding to water.

[0093] Replacing the Diethyl Ether with Another Solvent

[0094] Before using the sol, as stated previously, diethyl ether shouldbe replaced with another solvent, for example ethanol.

[0095] This replacement can take place easily, as shown on FIG. 1c, byevaporation of the diethyl ether, preferably under reduced pressure,addition of an appropriate solvent and concentration until a volumeclose to the initial volume V_(si) is obtained.

[0096] The final sol obtained shows the distribution of the followingsilicon species (molar composition):

[0097] T¹=3.9% T²=54.7% T³=41.4%

[0098] The condensation rate Tc of the silicon species is 0.79%.

[0099] Stability Test

[0100] The sol is kept for 18 months at 4° C. temperature.

[0101] The composition of the sol is then analysed again.

[0102] The distribution of the silicon species (molar composition) is:

[0103] T¹=2.6% T²=53.4% T³=44%

[0104] The condensation rate Tc of the silicon species is 0.80%.

[0105] The very high stability of the sols according to the inventioncan be observed.

[0106] Shaping and Condensing the Materials

[0107] Massive Materials

[0108] The sol is poured into a polypropylene mould.

[0109] This mould is then covered with an aluminum sheet pierced withtwo 0.5-diameter holes.

[0110] The sample is then placed in a stove.

[0111] The temperature of the stove is typically 60° C.

[0112] At that temperature, the sol has gellified and begins to shrinkafter 16 hours.

[0113] After four days, the xerogel is brought out from the stove.

[0114] The volume of the samples thus prepared is a few cubiccentimeters. They can be polished for an optical application.

[0115] Thin films:

[0116] The films are deposited by centrifugation.

[0117] The viscosity of the sol is adjusted after introducting of theorganic additive (if present). The volume of the sol is brought to alevel smaller than V_(si) for a viscous sol and thick films, and it canbe diluted for reducing viscosity and obtaining thinner films.

[0118] The viscosity of the sol (silicon concentration) and the rotationspeed of the substrate enable to vary the thickness of the films.

EXAMPLES

[0119] [Si] 7 moles/l—substrate 1300 rpm

[0120] thickness of the film obtained: 7 μm

[0121] [Si] =0.2 moles/l—substrate: 1000 rpm thickness of the filmobtained: 85 nm

[0122] The sols are deposited on different substrates: mineral glass,gold-coated mineral glass, double face polished crystalline silicium.

[0123] The films are then condensed at different temperatures, from 70to 130° C.

Comparative Example

[0124] A sol is prepared according to the method described in the patentFR 2 704 851. 0,1 mole MTEOS (methyltriethoxysilane) is mixed with 0.3mole water (i.e. 5.4 cm³) whose pH is adjusted to 2.5 by addinghydrochloric acid, and 0.3 mole ethanol (i.e. 17.5 cm³). The mixture isstirred for a few hours, then it is distilled under primary vacuum (10⁻³mm Hg or 10-3 torr) until a volume identical with that of the initialMTEOS is obtained. Thus, we obtain a syrupy sol showing a concentrationof 5.02 moles/l of silicon atoms.

[0125] The final pH of this sol ranges between 3 and 4.

[0126] This sol, obtained by stoichiometric hydrolysis, has aconcentration rate Tc in the order of 0.58 smaller than that of the solaccording to the invention. The silicon species (units) present in themedium are as follows: T¹ = 34.9% T² = 57.4% T³ = 7.7% (molarcompositions)

[0127] The sol is stored at 4° C. for 11 months.

[0128] New measurements show that the condensation rate Tc has increasedfrom 0.58 to 0.90.

[0129] The silicon species present are T¹ = 0.6% T² = 29.5% T³ = 69.9%

[0130] Moreover, particles have precipitated, which calls for filtrationbefore using the sol.

[0131] Mechanical Characterisation

[0132] The results obtained relate to 5 to 7-μm thick films.

[0133] These measurements consist in indenting the several micron-thicklayers over a few hundred nanometers in depth.

[0134] Nano-indentation enables to obtain characteristic values of themechanical behaviour of the films.

[0135] Er (reduced Young modulus) and H (plastic hardness) are measured.Film (hardening Solubilised element conditions) Er H (MPa) Ex. 1 (1 h30 - 130° C.) 2.35 180 (3 days - 130° C.) 4.08 650 Ex. 1 + SiO₂ (OSCALL1122 A8 55% Shokubai) of the dry extract (1 h 30 - 130° C.) 6.49 450 ofthe sol (3 days - 130° C.) 9.46 1200  Comparative ex. 100° C. 3.1 110-140

[0136] Incorporation of Additives

[0137] The solubility of a few molecules has been compared in two typesof MTEOS sols. Sol, comparative example Sol, example 1 % dry % dryextract extract Molecules Molar ratio mass Molar ratio mass Photochromiccompounds Films SO1 10/1000 (5.8%) 50/1000 (29%) SO6  1.8/1000 (5.8%) 6/1000  (4.2%) precipitation in sol precipitation during drying SO4 2/1000  (1.2%) DTE 1  2/1000 (1.7%) 10/1000  (8.7%) precipitation insol Other molecules Xerogel Ruthenium 3.5 × 10⁻⁴ (0.40%) 3.5 × 10⁻⁴ (0.40%) phtalocyanine Precipitation Tin naphtaocyanine 1.3 × 10⁻⁴(0.32%) 10⁻³  (2.1%) Spin-transition iron Insoluble 2 × 10⁻⁴  (0.21%)complex

[0138] Regardless of the molecules, the sol according to the inventionprovides far better solubility.

[0139] The sols of the invention enable to introduce molecules that hadnever been incorporated to that type of sol so far.

[0140] Determining the Condensation Rate

[0141] The condensation rate of the phases is obtained by RMN ²⁹Si.

[0142] This technique enables to differentiate all the silicon atomsfrom various environments.

[0143] In the case of a totally hydrolysed sol, the species present inthe medium are as follows: Denomination R¹Si (OH)₃ T⁰ R¹Si (OH)₂—0Si T¹R¹Si (OH)(OSi)₂ T² R¹Si (OSi)₃ T³

[0144] The condensation rate Tc is the ratio of the number of formedsiloxanes to the number of maximum siloxanes.

Tc=(1kT ¹+2kT ²+3kT ³)/3

[0145] where kT¹, kT² and kT³ are respectively the mola:r fractions inspecies T¹, T² and T³.

[0146] Examples of Sols Containing an Epoxytrialkoxysilane MonomerProceeding as previously, the following sols have been prepared: Monomerprecursors % molar ETEOS DMDMOS MTMOS GLYMO Segregation conditions Sol 1100 — — — 4 weeks at 25° C. and 1 week at 4° C. Sol 2 — 25 — 75 Nosegregation Sol 3 — 42 — 58 3.5 days at 25° C. and half a day at 5° C.Sol 4 — 50 — 50 3.5 days at 25° C. and half a day at 5° C. Sol 5 — 75 —25 3.5 days at 25° C. and half a day at 5° C. Sol 6 — — 25 75 Nosegregation Sol 7 — — 50 50 No segregation Sol 8 — — 75 25 1 week at 4°C.

1. A method for preparing an organo-silicon soil, characterised in thatit comprises: a) hydrolysis of an initial volume V_(si) of a precursormaterial containing at least an organo-silicon monomer precursor withformula: R¹ _(n)si (OR²)_(4-n)  (i) in which the radicals R¹, identicalor different, represent an alkyl group, an aryl group, a vinyl group orH, the radicals R², identical or different, represent H or an alkylgroup, and n is an integer varying from 1 to 2, and n=2 if R¹ representsH, with a water quantity such as$\frac{x\quad H_{2}O}{x{Si}} \geq 10$

and, with a possible quantity of an organic solvent such that$0 \leq \frac{x{Solvent}}{x{Si}} \leq 8$

where x H₂O, x si and x solvent represent, respectively, the number ofmoles of H₂O, si and solvent present: and under the condition that when${\frac{x\quad H_{2}O}{x{Si}} = 10},{{x{Solvent}} = 0},$

to obtain a hydrolysate of the precursor material; b) concentration ofthe hydrolysate down to a volume substantially equal to the initialvolume V_(si); c) leaving the concentrated hydrolysate until a distinctaqueous phase and a distinct organo-silicon phase are obtained, and d)separation and collection of the organo-silicon phase.
 2. A methodaccording to claim 1, characterised in that, R¹ represents a methyl,ethyl, phenyl, substituted phenyl radical, preferably substituted withnon-polar groupings, or a vinyl radical; R² represents a C₁ to C₇ alkylradical; and n is 1 or 2, preferably 1 when R¹ does not represent H. 3.A method according to claim 1 or 2, characterised in that the quantityof hydrolysis water is such that$10 \leq \frac{x\quad H_{2}O}{x{Si}} \leq 20$


4. A method according to any of claims 1 to 3, characterised in that theprecursor material comprises moreover at least another monomer precursorselected among the epoxytrialkoxysilanes.
 5. A method according to claim4, characterised in that the epoxytrialkoxysilanes comply with theformula:

in which: R is a C₁-C₆ alkyl group, R′ represents H or CH₃, a is aninteger from 1 to 6, and b is equal to 0, 1 or
 2. 6. A method accordingto claim 5, characterised in that the epoxytrialkoxysilane isγ-glycidoxypropyltrimethoxysilane.
 7. A method according to any ofclaims 4 to 6, characterised in that the epoxytrialcoxysilane(s)represents 50% or less of the monomer precursors.
 8. A method accordingto any of claims 5 to 7, characterised in that the monomer precursor offormula (I) is selected among methyltriethoxysilane,ethyltriethoxysilane and their mixtures.
 9. A method according to any ofthe previous claims, characterised in that the hydrolysate has a pHranging from 3 to
 10. 10. A method according to any of the previousclaims, characterised in that a predetermined quantity of mixedcolloidal silica SiO₂ is incorporated to the organo-silicon phaserecovered.
 11. A method according to any of the previous claims,characterised in that it also comprises (e) a drying step of therecovered organo-silicon phase.
 12. A method according to claim 11,characterised in that the drying step comprises the addition of asolvent with a boiling point above 100° C. at atmospheric pressure or asolvent forming an azeotrope with water and evaporation of the solvent.13. A method according to claim 11, characterised in that the dryingstep comprises an extraction with a hydrophobic solvent.
 14. A methodaccording to claim 13, characterised in that the hydrophobic solvent isdiethyl ether.
 15. A method according to claim 14, characterised in thatthe diethyl ether is replaced with an appropriate solvent before usingthe organo-silicon sol.
 16. An organo-silicon sol made of siliconspecies T¹, T² and T³, characterised in that the molar fraction ofspecie T² is equal to or greater than 50%, preferably 60%, and thecondensation rate Tc is equal to or greater than 0.65.
 17. Anorgano-silicon sol according to claim 16, characterised in that it isdeprived of water, as determined by the absence of peaks correspondingto water by RMN ¹H.
 18. An organo-silicon sol according to claim 16 or17, characterised in that it contains at least one solubilisedphotochromic compound.
 19. An organo-silicon sol according to any ofclaims 16 to 18, characterised in that it has been obtained by themethod according to any of claims 1 to
 15. 20. A film or xerogelobtained from a polyorgano-silicon sol according to any of claims 16 to19.